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"A 2G-world human with the same mass as an Earth human might be half the height, but otherwise the same proportions."

(another writer) "I don't think a 2G human would have the same proportions -- I'd expect short, thick bones, like a human whose limbs were just shortened without being thinned. Barely any neck (heads are heavy)."

QUOTE: The respected zoologist D’Arcy Wentworth Thompson once speculated about the effects of gravity on evolution. “Were the force of gravity to be doubled,” Thompson declared, “our bipedal form would be a failure, and the majority of terrestrial animals would resemble short-legged saurians, or else serpents. Birds and insects would suffer likewise, though with some compensation in the increased density of the air. On the other hand, if gravity were halved, we should get a lighter, slenderer, more active type, needing less energy, less heat, less heart, less lungs, less blood. Gravity not only controls the actions but also influences the forms of all save the least of organisms.”

So you're just taking the inverse of 2.4? Why?
I'm also impressed by the result to three decimal places, which is precision to a couple of millimetres. Given that here on Earth adult humans of my acquaintance differ in height by more than half a metre, and in weight by more than 150kg, I doubt we can make any confident prediction about human height vs. weight in other gravitational environments.

So you're just taking the inverse of 2.4? Why?
I'm also impressed by the result to three decimal places, which is precision to a couple of millimetres. Given that here on Earth adult humans of my acquaintance differ in height by more than half a metre, and in weight by more than 150kg, I doubt we can make any confident prediction about human height vs. weight in other gravitational environments.

Reducing height would also be accompanied by reductions in other dimensions.

LATE. You are right that function would be lost. My point, which I admit was ill made, is that high gravity planets could be bad places for highly intelligent life, for a variety of reasons. My apologies, I am thinking this out on the fly.

I'm also impressed by the result to three decimal places, which is precision to a couple of millimetres. Given that here on Earth adult humans of my acquaintance differ in height by more than half a metre, and in weight by more than 150kg, I doubt we can make any confident prediction about human height vs. weight in other gravitational environments.

It is a bad habit of mine. I like statistics and math, and it runs away from me at times.

My point is really that there is no necessity to scale in precise (or even approximate) inverse proportion to gravity. We have very large animals, and very tall animals, that operate quite effectively in Earth's gravitational field. So increased weight and increased hydrostatic pressure differences aren't show-stoppers - gravity demonstrably doesn't force biology to occupy a very narrow scale range.

Don't flatfish, such as founder and halibut, have a vertically squashed form compared to most other fish? They seem to function ok...

They're actually squashed a little from side to side, and lie on their sides on the bottom - considerable anatomical rearrangement is required, including a highly distorted skull to get both eyes on the same side of the body.

If you compressed a human only on the vertical axis, you'd change the mechanics of all the long bone muscles, among other things - nothing would work properly. But it turns out that Roger E. Moore wants to shrink in all dimensions, which at least retains the geometry. There are still problems, though, because there are some things you can't shrink successfully without rewriting cell biology - the diameter of the capillaries and the thickness of the alveolar membrane for instance. So it turns out that metabolic factors scale with different exponents from geometry. While mechanical aspects scale to body mass with exponents that are typically integer multiples of 1/3, metabolic components (like blood flow and gas exchange) have exponents that are odd multiples of 1/4, 1/6, 1/8 or 1/12. This wouldn't necessarily be a problem, but it would mean that a half-size human can't be a half-scale model of a full size human - all sorts of anatomical differences would be necessary. And I really do wonder what revisions would be necessary to make a half-size human baby work like a conventional human baby.
So my point is that you can't ever just make a scale model - anatomy needs to be rewritten, either grossly or at the cellular level or both. And given the scale range of organisms we see around us on the Earth, and the multiple physiological differences that allow that scale range to occur, I can't see why there would ever be a simple scale law relating to gravity - Nature is smarter and more complicated than that.

They're actually squashed a little from side to side, and lie on their sides on the bottom - considerable anatomical rearrangement is required, including a highly distorted skull to get both eyes on the same side of the body.

If you compressed a human only on the vertical axis, you'd change the mechanics of all the long bone muscles, among other things - nothing would work properly. But it turns out that Roger E. Moore wants to shrink in all dimensions, which at least retains the geometry. There are still problems, though, because there are some things you can't shrink successfully without rewriting cell biology - the diameter of the capillaries and the thickness of the alveolar membrane for instance. So it turns out that metabolic factors scale with different exponents from geometry. While mechanical aspects scale to body mass with exponents that are typically integer multiples of 1/3, metabolic components (like blood flow and gas exchange) have exponents that are odd multiples of 1/4, 1/6, 1/8 or 1/12. This wouldn't necessarily be a problem, but it would mean that a half-size human can't be a half-scale model of a full size human - all sorts of anatomical differences would be necessary. And I really do wonder what revisions would be necessary to make a half-size human baby work like a conventional human baby.
So my point is that you can't ever just make a scale model - anatomy needs to be rewritten, either grossly or at the cellular level or both. And given the scale range of organisms we see around us on the Earth, and the multiple physiological differences that allow that scale range to occur, I can't see why there would ever be a simple scale law relating to gravity - Nature is smarter and more complicated than that.

This is a better way of making my earlier point that high gravity worlds are not friendly to land based intelligent life. Thank you for that. I was unable to get that out properly.

Here is an amusing calculation on the effects of a human tripping and falling on LHS 1140b. Assume the human's face is 5.5 feet off the ground, = 1.6775 m. On Earth, a faceplant would result in a collision with the ground at a speed of 20.63 km/hr, if I got the math right. However, the acceleration of gravity on LHS 1140 is 23.7 give or take 2.7 m/s^2, not 9.8067 (per Kristo Ment et al. 2018). If I got it right, a faceplant would give a final nasal-ground collision speed of 32.1 km/hr. Quite a bit more. On Earth, this would be like falling from a height of 4 meters, ouch.

Now if (a big if) there is value in survivability in tripping and hitting the ground at 5.736 m/s (20.63 km/hr), and if bipedal aliens are made of the same stuff we are, a native of LHS 1140 could be only 0.694 m tall and no more to hit the ground at a survivable velocity.

I think you just worked out how fast a detached head would hit the ground if dropped from head height.

The real calculation needs to take into account the fact that the body topples - you need to know the centre of gravity and moment of inertia of a human body, and follow the gradually increasing rate of rotation as the velocity vector rotates to align with the gravity vector.

I think you just worked out how fast a detached head would hit the ground if dropped from head height.

The real calculation needs to take into account the fact that the body topples - you need to know the centre of gravity and moment of inertia of a human body, and follow the gradually increasing rate of rotation as the velocity vector rotates to align with the gravity vector.

It's difficult to model a human fall - the reason adults face-plant is usually because we enter the fall with some linear velocity, which translates to angular velocity, shortening the time to impact while increasing the impact velocity.
But taking a naive model of a rigid stick of length L that starts to topple from zero angular velocity at some angle of tilt, we can see a problem with scaling down height to reduce impact velocity.
Impact angular velocity varies with the square root of g/L, which implies impact velocity for a given point on the stick will vary with the square root of gL. But the time to topple varies with square root of L/g. The shorter you are, the slower your impact but the less time you have to react in order to save yourself or cushion your fall - we've all seen how quickly tripping children hit the ground, but how relatively uninjured they are in the process. Adults who injure themselves after tripping have typically gone down fast because of initial velocity, or have failed to react in time (because of impaired neurology or mobility).

So, in terms of avoiding injury under high gravity, there are competing pressures on the height of the toppling person - a trade-off of time versus velocity.

Being an older person who dislikes falling, I have discovered the tactic of immediately trying to crouch when I am off balance, to reduce the consequences of impact. It works.

A bulky astronaut suit would not help. Moonwalkers have fallen but that was on the Moon. The taller the being on LHS 1140b, the better protected it should be from injury and the more stable (big footed or multifooted).

Part of me wants to traipse gaily down the familiar path of advocating centaurine or elephantine intelligent life, but lately I suspect I am missing something important in doing this. The complexity of the infrastructure of an intelligent being grows vastly more complex under high gravity. More of the body's mass would be devoted to support, more to pumping fluids up against gravity, more to movement, and so on. A brain needs protection, nutrients, and additional mass beyond that for autonomic functions like breathing and necessary things like eating. The brain space required to sustain a 'dumb' creature on land seems enormous, regardless of morphology. Would a cerebrum be likely to be smaller as a result?

Could the size and number of alien animals be affected by planetary size and gravity? Thoughts are welcome.

Take a low-gravity world and a high-gravity world, each with life under Earthlike conditions. The low-gravity world is likely to be smaller in mass and radius than our Earth, if not lower in density, and the high-gravity world likely to be more massive and larger (and denser) than Earth. The low-gravity world will likely have a much smaller surface area than Earth, and the high-gravity planet a larger surface area. As radius increases, surface area increases by the square (power of 2), so even a modest increase in radius could give a planet a significantly larger area than before. Volume (and thus mass, assuming equivalent densities) increases by the cube, driving up gravity.

For the moment we ignore the ratio of land to sea, though a small sea-less world could, like Mars, could have the same land area as does Earth. (Mars in fact does.)

On the low-gravity world, species can grow to greater heights than on Earth and move faster, as running and hopping are better enabled by low gravity so long as skeletal and body structure protect fast-moving creatures who crash into things or fall. The opposite occurs on the high-gravity world, as the effects of weight and mass reduce both maximum creature size and likely maximum speed. A fast-moving high-gravity creature builds up a lot of momentum and force, which will likely injure it if things go wrong. This applies even to sea creatures.

The key point is, on a low-gravity world with relatively small surface area, species can cross great distances and mark out large territories. This applies to predators and prey alike. However, a small planetary surface area means less area in which to feed, either on vegetation or on other species. The small feeding area immediately restricts the maximum number of creatures per species.

In addition, because movement is improved, species all over the planet—unless separated by seas, deserts, ice, or mountains—could easily encounter one another. Few species will be confined to specific areas unless scarcity of water or food is a factor. The same genus or species of animal might be encountered with equal likelihood anywhere on the world that is accessible to all. Larger prey are larger targets for wide-ranging predators. Would not the disadvantages of size, in both attracting attention from predators and requiring more nutrition from a limited land area, restrict the maximum size of animals in general? Would not the total number of species on a smaller world, due to competition over a small area, also be fewer than on Earth as well? And the same for the total number of animals of each species? Kind of like finding pygmy elephants and hippos on islands.

On high-gravity worlds, however, with vast land areas and slower movement, localization of species would seem to be the norm, creating enormous numbers of species and greater numbers of creatures per species the world over. Creature size is restricted by gravity, but because movement is hindered, the available feeding area per creature is also restricted to a degree, which will affect species numbers unless the food source is thick and widespread.

I think you just worked out how fast a detached head would hit the ground if dropped from head height.

The real calculation needs to take into account the fact that the body topples - you need to know the centre of gravity and moment of inertia of a human body, and follow the gradually increasing rate of rotation as the velocity vector rotates to align with the gravity vector.

Grant Hutchison

Giraffes don't know that (they're only stupid extra tall horses) and just continue to live... sometimes they fall hitting the ground with their head and just die :no:

Giraffes don't know that (they're only stupid extra tall horses) and just continue to live... sometimes they fall hitting the ground with their head and just die :no:

Giraffes fall over surprisingly infrequently, and I'd be surprised if one hit its head in doing so, simply because it takes them so long to fall that they've got plenty of time to lift their small heads away from the ground by curving their long necks.
Do you have a reference for giraffes sustaining head injuries from falling? I'd be very interested in how it happened.

Giraffes don't know that (they're only stupid extra tall horses) and just continue to live... sometimes they fall hitting the ground with their head and just die :no:

Originally Posted by grant hutchison

Giraffes fall over surprisingly infrequently, and I'd be surprised if one hit its head in doing so, simply because it takes them so long to fall that they've got plenty of time to lift their small heads away from the ground by curving their long necks.
Do you have a reference for giraffes sustaining head injuries from falling? I'd be very interested in how it happened.

You seem not to have noticed the word "no" at the end of Barabino's statement... I've bolded it to help you...

Giraffes fall over surprisingly infrequently, and I'd be surprised if one hit its head in doing so, simply because it takes them so long to fall that they've got plenty of time to lift their small heads away from the ground by curving their long necks.

As an older person who sometimes falls down, I am extremely jealous of giraffes.

You seem not to have noticed the word "no" at the end of Barabino's statement... I've bolded it to help you...

Thanks for the kind effort to help, but I did actually read to the end of the post. It's impossible to tell from context if the ":no:" means "no, everything I just wrote is counterfactual" or "oh no, poor giraffe". I took it to mean the latter, since the former would be ... odd. But if it's the former, then it seems Barabino and I are in complete agreement, and I just added a detailed supporting argument. (A shame if so, because I wanted to know more about those falling giraffes - the fact that they do fall so rarely is something of a neurological puzzle.)

... (A shame if so, because I wanted to know more about those falling giraffes - the fact that they do fall so rarely is something of a neurological puzzle.)

Birds also crash into things too .. sometimes with catastophic consequences .. I'm puzzled about this as well, given their mastery of flight. Errors in perceptual judgement seems to be in-common across all of Earth's lifeforms(?)
There are no 'golden rules' however .. but always uncertainties ...